Referring to FIG. 1, there is provided a diagram of a conventional holographic ROM system. As shown in FIG. 1, the conventional holographic ROM system includes a holographic disk 120 as a recording medium for storing holograms, a pick-up module 100 for optically reading the holograms in the holographic disk 120 and then producing electrical signals in response to what has been read, a signal processing unit 150 for processing the electrical signals transmitted from the pick-up module 100, and a control unit 140 for controlling a first and a second actuator 102 and 116 and a spindle motor 130.
A reconstruction reference beam, generated from a light source 104 of the pick-up module 100, is reflected by a reflecting surface of a double sided reflecting section 106 to reach a mirror 108. The mirror 108 is, for example, an actuated mirror controlled by the control unit 140 in order for the reconstruction reference beam to reach the holographic disk 120 at an appropriate incidence angle. The reconstruction reference beam reflected by the mirror 108 reaches the holographic disk 120 through a reducing lens 110 at an incidence angle corresponding to that of a reference beam used in a data storing process, wherein the holographic disk 120 is rotated by the spindle motor 130 at a predetermined speed.
A part of the reconstruction reference beam incident upon the holographic disk 120 is diffracted to make a reconstructed signal beam. The reconstructed signal beam reaches the other reflecting surface of the double sided reflecting section 106 through a first lens 112, moved by the second actuator 116, and a second lens 114. Next, the reconstructed signal beam is reflected by the other reflecting surface of the double sided reflecting section 116 to reach a detector 118. The detector 118 transmits electrical signals to the signal processing unit 150 in response to the reconstructed signal beam.
In the holographic ROM system, a laser diode is generally used as the light source 104. The laser diode continuously emits heat during its operation, and a power supply of the holographic ROM system also emits heat. Therefore, due to such heat dissipation from the laser diode, the power supplying device and the like, internal temperature of the holographic ROM system, i.e., operating temperature of the laser diode, rises.
Referring to FIG. 2, there is illustrated a graph showing a wavelength variation of a laser beam generated from the laser diode, which depends on the operating temperature of the laser diode. When the operating temperature is 10° C., the wavelength of the laser beam generated from the laser diode is about 648 nm. However, as the operating temperature rises, the wavelength of the laser beam goes up. When the operating temperature is 70° C., the wavelength of the laser beam is about 658 nm, which is greater than the wavelength at 10° C. by 10 nm.
Further, in the conventional holographic ROM system, the data stored in the holographic disk 120 is reconstructed by a phase-conjugate readout method. Therefore, if the wavelength of the reconstruction reference beam generated from the light source 104 in a reconstruction process is considerably varied owing to change in the operating temperature of the laser diode as described above, the wavelength of the reconstruction beam may become greatly different from that of the reference beam used in the data storing process so that a level of the reconstructed signal beam obtained by the diffraction from the holographic disk goes down abruptly and, in a more serious case, such a situation as the reconstruction is impossible may occur.
Accordingly, the conventional holographic ROM system suffers from a drawback that the reliability of the reconstructed signal beam is degraded because of a high wavelength variation rate of the reconstruction reference beam generated from the light source to the operating temperature of the light source.